Many conventional semiconductor packaging techniques contemplate the use of an encapsulant that covers at least a portion of the die. One common approach is to form a package by molding a plastic encapsulant material around the die. In other arrangement, epoxies or other adhesive materials may be dispensed over at least the active surface of the die. At the same time, some integrated circuits have sensitive regions that can be adversely affected by stresses caused by the adherence of an encapsulant (e.g., mold compound or epoxies) to the die. Typically, these sensitive regions are found on an active surface of the die.
Stresses in molded integrated circuit packages occur mainly from thermal mismatch between the different materials in the packages. Piezo-resistive test chip measurement showed that the stresses depend on a number of factors, such as package type, die thickness, die size, die pad size, die attachment type, and mold compound type. Imbalance in package configuration also causes bending stresses on the die. Typical assembly process induced stresses are about −10 MPa or more from the die attach, and about −300 MPa from the molding (postmold cure) process. Molding-induced stresses may cause parametric shift including hysteresis after preconfiguration, smearing of unpassivated and passivated aluminum lines, dielectric cracks, pasivation cracks, and the like. FIG. 1 schematically illustrates molding-induced compressive stresses and in-plane shear stresses in a conventional integrated circuit package.
A number of techniques have been used and/or proposed to reduce or prevent such molding-induced stresses. Conventional solutions include, among others, cavity packages, low stress mold compounds, redesigned leadframes, and soft overcoats. Cavity packages are expensive solutions due to the high cost of pre-molded cavity packages and cavity ceramic packages, yet presenting stresses from die attachment about −10 MPa or more. Using a low stress mold compound achieves only a small extent of stress reduction due to the limitation of flexibilizers, filler type, and filler content that need to be altered. Redesigning leadframes includes use of anchors, selective plating, leadframe micro etching, and the like to enhance interfacial adhesion, whose effect on stress reduction is limited. Soft overcoats such as silicon or polyimide are used for glob top or in photo-sensitive coatings to act as stress buffers. However, such coatings can only reduce in-plane stresses and thus compressive and bending-induced stresses are still present. For example, stresses about −40 to +80 MPa from die attachment and mold compound are still present in some applications.